Environmental concerns have motivated the implementation of emission requirements for internal combustion engines and other combustion systems throughout much of the world. Catalytic converters implemented in exhaust gas after-treatment systems have been used to eliminate many of the pollutants present in exhaust gas; however, a filter is often required to remove particulate matter, such as, for example, ash and soot. Wall-flow particulate filters, for example, are often used in engine systems to remove particulates, such as soot and ash, from the exhaust gas. Such particulate filters may be made of a honeycomb-like substrate with parallel flow channels or cells separated by internal porous walls. Inlet and outlet ends of the flow channels may be selectively plugged, such as, for example, in a checkerboard pattern, so that exhaust gas, once inside the substrate, is forced to pass through the internal porous walls, whereby the porous walls retain a portion of the particulates in the exhaust gas. Particulate capture by the porous walls can occur in two different stages: at first, inside the porous wall (deep-bed filtration), and later, on the porous wall in the flow channels (cake-bed filtration).
In this manner, wall-flow particulate filters have been found to be effective in removing particulates, such as, for example, ash and soot, from exhaust gas. However, the pressure drop across the wall-flow particulate filter increases as the amount of particulates trapped in the porous walls and channels increases. For a filter that is not conditioned (e.g., that does not have enough of an ash layer to stop particulate matter from penetrating the porous filter walls), there is generally a rapid increase in pressure drop during the initial deep-bed filtration stage (in which typically less than 1 g/L (grams per liter) of particulate is captured in the filter), followed by a gradual increase in pressure drop with particulate loading during the cake-bed filtration stage. The increasing pressure drop results in a gradual rise in back pressure against the engine, and a corresponding decrease in the performance of the engine.
A filter's geometric and microstructural properties can not only influence the filtration efficiency (FE) of a filter, but can also affect the initial pressure drop during the deep-bed filtration stage. For example, the initial increase in pressure drop is more influenced by a filter's microstructural properties in the deep-bed filtration stage than by its geometric properties, which tend to have more of an effect in the cake-bed filtration stage. It may be desirable, therefore, to provide microstructural properties that not only result in high filter FE prior to any particulate loading of the filter (i.e., when the filter is clean), but also achieve a relatively low deep-bed filtration related initial back pressure increase, thereby minimizing the filter's pressure drop while it is clean and during the initial stage of deep-bed filtration.
Moreover, as will be explained further below, it may be desirable to provide a filtration structure in which the pressure drop across the filter remains relatively low prior to soot loading of the filter (e.g., when the filter is clean), as well as during the initial stage of soot loading (i.e., during deep-bed filtration).